Many work vehicles have elongate members or linkages that are controlled by hydraulic actuators. When the hydraulic actuators are filled with fluid, typically under the control of hydraulic spool valves, the members move with respect to the work vehicle. When the spool valves are closed, hydraulic fluid stops flowing to the hydraulic actuators and the members stop moving.
When the members are particularly long, or are connected to one another using mechanical linkages that are loose, the relatively sudden closing of the hydraulic valves causes the hydraulic actuators to abruptly stop moving. The sudden halt to the motion of the hydraulic actuators does not immediately stop the motion of the members themselves. Typically, and especially for elongate members or linkages such as backhoe attachments connected to work vehicles such as tractors, the members flex and oscillate back and forth for a period of four or five seconds before finally coming to a halt.
When the work vehicle is being used for precise operations such as digging a very narrow trench that is just the width of a backhoe bucket attached to the end of the elongate member, the operator must wait until the oscillations cease before lowering the bucket into the narrow trench. This delay causes a significant reduction in work vehicle productivity. In many cases the operator must reposition the elongate members using the hydraulic valve once they have stopped oscillating.
This oscillation is due to the spring-like behavior of the elongate members and their hydraulic actuators. When the hydraulic valve is closed, the hydraulic actuators stop moving instantly. The inertia of the elongate members extending outward from the vehicle tends to keep the elongate members moving. As the elongate members attempt to keep moving, the lower portion of the members (i.e. the end that is pivotally attached to the work vehicle) butts up against the now-stopped hydraulic actuators, causing a sudden spike in hydraulic pressure in the actuators. The actuators, which are fixed in their extended position since the valve has closed, resist the force applied to them by the elongate members and stop essentially all motion. As a result, the free end of the elongate member flexes, and its kinetic energy is converted into potential energy in the form of any elastically bent elongate member, pressurized hydraulic fluid in the hydraulic actuator, and perhaps slightly elastically expanded hydraulic hoses and actuators.
As the elongate members slow to a halt and their kinetic energy is converted into potential energy, the potential energy is then released and converted back into kinetic energy again as the elongate members begin to swing back in the opposite direction.
This reversal of direction causes the same process to occur in the reverse direction. The actuators, rebounding from the now-fixed actuator, increase in speed until they begin to pressurize the fluid in the actuator in the other direction, at which point they begin slowing down as the pressure increases in the actuator until the elongate members are again stopped. The elongate members then begin moving back in the original direction once again. This back and forth motion of the elongate members continues for as many as five seconds until finally its energy is completely dissipated, at which point they slow to a halt.
Depending upon the inertia and momentum of in the elongate members, the pressure spike in hydraulic actuators that move the elongate members can be substantial. Oftentimes, the kinetic energy pressure spike is several times as large as the maximum operational pressure of the hydraulic actuators. Previous attempts to reduce this unwanted oscillation have capitalized on this difference between the working pressure and the sudden pressure pulse experienced by the hydraulic actuator. In these prior art efforts, a hydraulic relief valve is coupled to the ports of the hydraulic actuators to permit the escape of fluid thereby damping the oscillations. This relief pressure is significantly greater than the operating pressure of the hydraulic actuators, typically on the order of 2500 psi. By comparison, the actual operating pressure of a backhoe swing cylinder—the hydraulic actuator that pivots a backhoe attachment—is typically 900 psi.
One of the problems with relying only relief valves to reduce elongate member oscillation is the fact that the relief valve pressure setting must of necessity be greater than the typical operating pressure of the hydraulic actuator. Since pressure relief valves are continuously connected between a hydraulic actuator and a low-pressure hydraulic tank or reservoir their relief pressure setting must be above that of the maximum desired operating pressure, or that operating pressure would never be reached. It is for this reason that the overpressure relief valves cannot be relied upon to reduce the oscillation of the elongate members, due to this sudden pressure spike when the hydraulic valve is closed.
Since the relief pressure is set so high, the overpressure relief valves will only pass hydraulic fluid (and dissipate the members' potential energy) to a limited extent. A substantial amount of the potential energy remains stored in the system, however, and can only be dissipated by the oscillations of the elongate members. Indeed, oscillations can occur when the pressure in the actuator is as low as 300-400 psi, well below the pressure at which the relief valves open.
For this reason, a simple pressure relief valve cannot function to substantially reduce the oscillations of the elongate member.
A swing-damping system is shown in co-pending patent application Ser. No. 09/661,348 filed on Sep. 14, 2000, entitled “Hydraulic System and Method for Regulating Pressure Equalization to Suppress Oscillation in Heavy Equipment”, and assigned to Case Corporation (assignee of the present application). In that application, a hydraulic circuit for damping unwanted oscillations or swing of an elongate member of a work vehicle is also shown.
In the system of the '348 application, a backhoe is shown with a purely hydraulic swing damping circuit coupled to and between a bi-directional hydraulic valve (called a “boom swing valve”) and two bi-directional dual-ported boom swing cylinders that are coupled between a backhoe attachment (the elongated members in this application) and a work vehicle to which the attachment is pivotally coupled.
This swing-damping circuit of the '348 application responds to pressure in the boom swing cylinder that is generated by the boom as it slows down and decelerates, pushing against the actuator. This state occurs whenever the boom swing valve is closed so far that it does not permit sufficient fluid out of the cylinder to accommodate the already-moving boom swing cylinder piston. A pressure pulse is generated in the boom swing cylinders when the hydraulic valve controlling fluid flow to and from those cylinders is suddenly closed and the backhoe boom impacts the suddenly stopped boom swing cylinder piston. This sudden spike in pressure, typically greater than the ordinary operating pressure of the cylinder and hydraulic valve controlling the cylinder, applies a hydraulic pressure to one side of a small hydraulic valve spool in a bypass or crossover circuit. This pressure spike causes the valve spool to shift and permits fluid to flow from the port of boom swing cylinder experiencing the sudden pressure spike to the opposing boom swing cylinder port. The spool and its associated hydraulic circuitry are arranged such that this additional flow path from one boom cylinder port to the other port closes shortly after it opens. Thus, for a short period of time after the hydraulic valve is closed, fluid is permitted to flow between the cylinder ports, and reducing excessively low pressure at the boom swing cylinder port used for initial acceleration, thus breaking the cycle of kinetic energy to potential energy to kinetic energy to potential energy oscillations.